This invention relates to an electric-oil pressure power steering system, and more particularly to an electric-oil pressure power steering system using an electronic switching means comprising a relay device having a smaller resistance loss in circuit and a semiconductor switching element of a smaller capacity, in which an output of an oil pump driven by an electric motor may change to be congruous with a vehicle speed at the only time of steering.
In general, larger steering power is required to operate a steering wheel of a vehicle at a stepping state or a lower speed state of the vehicle so as to park the vehicle and put it into a garage, and it is now widespread to provide a power steering system for a small-sized vehicle as well as a large-sized vehicle to thereby assist such larger steering power.
Such power steering system may be classified by the kinds of power source thereof into, for example, an oil-pressure type, an air-pressure type, an electric type and so on, and is constructed so that, as a component of the system, an oil pump for generating oil pressure as a power source of the power steering system is generally employed, the change of steering torque is mechanically detected to operate a control valve for controlling the oil pressure to a power cylinder, and the oil pressure energized by the operation of the control valve facilitates the movement of the power cylinder to reduce the steering power to be required by the steering wheel.
In the power steering system, it is important to set up suitable steering power and it is desirable that when the vehicle is running at a lower speed, the supply of power fluid which assists the rotation of the steering wheel is increased to thereby allow the steering wheel to rotate lightly and when the vehicle is running at a higher speed, the supply of the power fluid is reduced to thereby cause the steering wheel to rotate heavily so as to stabilize the steering wheel. In a conventional power steering system, an oil pump is rotated by means of belt driving from an engine shaft of an automobile to cause the oil pressure to assist steering and, therefore, there are the disadvantages that steering assistant force is applied in excess at a high speed and driving power is extremely consumed. Accordingly, it has been expected to develop a novel power steering system of an energy saving type. An electric-oil pressure power steering system of this invention can meet the needs of the times and overcome the technical problems above.
In any case, it is necessary to reduce the supply of the power fluid to cause the steering wheel to rotate heavily at a normal or high speed running state, thereby preventing steering torque from decreasing in excess, and to increase the supply of the power fluid to obtain sufficient steering torque when larger steering torque at, for example, a lower speed or stopping state, is required.